The objective of the work proposed in this application is to understand the role of the complement system in immune-mediated glomerulonephntis (GN). Rat models of GN will be studied, including the Heymann nephritis (HN) model of membranous nephropathy, the Thy-1 model of mesangial proliferative GN, and immune complex GN which models lupus, postinfectious and membranoproliferative GN. In HN, complement appears to be activated directly on the glomerular epithelial cell (GEC), leading to injury of this key component of the glomerular capillary wall and the development of proteinuria. Overall, evidence exists that systemic inhibition of complement activation will protect against disease in HN, yet, has a number of untoward effects. The first specific aim will examine the utility of targeting the potent complement inhibitor, Crry, specifically to the GEC. Recombinant chimeric molecules will contain Crry coupled to monoclonal antibodies targeting GEC antigens or complement receptor 2 targeting iC3b/C3d in immune complexes. Studies will be performed to model these inhibitors in vitro followed by studies in the PHN model in vivo. The second specific aim exploits the current capacity to examine gene changes in a massively parallel fashion. The underlying hypotheses in this aim are that genes relevant to the pathogenesis of complement-dependent glomerular disease models are measurably altered relative to control animals, and that studies in cultured cells are surrogates to events occurring in vivo. Studies will be performed using Affymetrix U34A arrays examining 7000 known rat genes. Individual and clustered gene changes occurring in response to complement-mediated GEC and mesangial cell injury in vitro and in vivo will be identified, allowing determination of which gene changes are complement-dependent, antibody-dependent, and shared between cultured cells and diseased glomeruli. The third specific aim will examine complement receptors and regulators that affect the glomerulus, either because of their direct presence in glomeruli (decay accelerating factor and factor H-related protein) or because of their influence on glomeruli by processing immune complexes (factor H). Studies will examine the expression of these proteins and their mRNA in experimental glomerular diseases, as well as the effect of altering their function in health and disease states. These studies will define the biological role of these complement regulatory proteins. Successful performance of the studies proposed here will provide insight into the role of the complement system in glomerular diseases, as well as potential strategies for treating these disorders